To the spies at GCHQ who set the brain-melting challenge, that seems to have been taken as proof that the puzzle was not quite difficult enough, so this year they have gone one better.

A charity quiz book compiled by code-breakers at the government listening post contains a competition which they insist is even harder than the last one. They say it will take “months” for someone working alone to solve the many stages that lead to the final answer.

The first two parts of the GCHQ puzzleCredit:
Penguin/GCHQ

The first part of the puzzle, consisting of a Sudoku-style grid containing letters rather than numbers, and a series of famous faces, is reproduced here, but anyone wanting to try the whole puzzle will need to buy the book, which goes on sale tomorrow.

Solving the first part of the quiz will unlock the next stage of the puzzle, but GCHQ is being characteristically secretive about how that happens. Last year the first part of the puzzle took competitors to a website where the rest of the questions appeared, but, asked if a similar system was being used again, a spokesman said: “We’re not saying. People will just have to buy the book.”

All profits from the sale of The GCHQ Puzzle Book will go to Heads Together, the mental health charity set up by the Duke and Duchess of Cambridge and Prince Harry.

The GCHQ building in Cheltenham

Last year’s quiz, which started with a grid-style puzzle printed in the GCHQ Director’s official Christmas card, attracted 600,000 entries from all over the world, though many more people attempted the puzzle without submitting formal entries.

It became such an international obsession that puzzlers joined forces in chat rooms and online forums to swap ideas and solutions to the various parts of the test, but most were defeated.

Only three entrants were deemed to have completed the puzzle to the satisfaction of the analysts who set it. Each received a special paperweight as a prize.

The prize for this year’s quiz is yet to be announced, but the closing date for entries is February 28, and if GCHQ are right, anyone hoping to solve it had better get started straight away.